The present invention relates to a new and improved construction of a smoke detector of the type comprising a radiation source operated in a pulsed mode, a radiation receiver arranged externally of the direct radiation region of the radiation source, the radiation receiver in the presence of smoke within the radiation region being impinged by scattered radiation and delivering output signals. Further, there is provided an evaluation circuit capable of triggering a signal when radiation pulses of the radiation source and output pulses of the radiation receiver are in coincidence.
Such type smoke detector has become known to the art for instance from U.S. Pat. No. 3,316,410. Here, a radiation source is controlled by a pulse transmitter and transmits briefly lasting radiation pulses. The evaluation circuit connected with the scattered radiation receiver is controlled by the pulse transmitter of the radiation source such that when receiving scattered radiation, it is only capable of delivering an output signal during the pulse phases of the radiation source. Spurious pulses, arising between the radiation pulses, are therefore blocked by the evaluation circuit and cannot lead to triggering a signal.
What is disadvantageous with this prior art construction is that spurious pulses, which randomly arise at the same time as the radiation pulses, can trigger a faulty signal.
To avoid this drawback it has already been attempted to connect with the evaluation circuit of a smoke detector, which operates with coincidence logic, an integrator or storage which first then delivers a signal if the evaluation circuit, within a certain time, has delivered a predetermined number of output pulses. Significant in this regard is U.S. Pat. No. 3,946,241, granted Mar. 23, 1976.
Yet, such type smoke detector, while being less prone to issuing a faulty signal, and therefore, having an improved operational reliability, nonetheless if there arise a number of spurious pulses in succession it is still possible for random ones of a number of these spurious pulses to coincide with the radiation pulses and therefore to cause tripping of a faulty signal.